1. Field of the Invention
The present invention relates to an optical distributor for splitting an input optical signal and outputting the split optical signals, and an optical distributing system using the optical distributor.
2. Description of the Related Art
In an optical network system for transmitting an optical signal to a plurality of devices connected to optical fibers, optical splitters (optical distributors) each for splitting an optical signal and transmitting the split optical signals to the devices are disposed at respective positions. The optical signal is input to desired devices by way of the optical splitters. For the optical splitters of this type, the following optical splitters have been known.
<Literature 1>
F. Auracher and H.-H. Witte, “Optimized layout for a data bus system based on a new planar access coupler”, APPLIED OPTICS, Vol. 16, Nos. 12, 3140–3142, December 1977
The literature 1 discloses a coupler for coupling and/or splitting an optical signal with respect to a trunk fiber (FIG. 1 of the literature 1). A connecting portion of the main fiber is shifted, and a splitting fiber is connected to the connecting portion. Thereby, the coupler is implemented.
<Literature 2>
Marshall C. Hudson and Frank L. Thiel, “The Star Coupler: A Unique Interconnection Component for Multimode Optical Waveguide Communications Systems”, APPLIED OPTICS, Vol. 13, No. 11, 2540–2545, November 1974
The literature 2 discloses an optical coupler for coupling and/or splitting an optical signal with respect to a main fiber (FIG. 1 of the literature 2). In the literature 2, a multi-to-multi optical coupler such as a star coupler is used. An optical coupler of a reflection type may be used as another form of the optical coupler (FIG. 2 of the literature 2).
An optical data way terminal device is disclosed in JP-A-62-014538. In the terminal device, an optical signal from a single optical fiber transmission path, which is arranged to have a looped shape or a linear shape, is split at a splitting portion having a variable splitting ratio.
An optical bus system including an optical fiber line and an optical coupler is disclosed in JP-A-63-001223. A splitting ratio of the optical fiber is gradually increased from a start end of the optical fiber line so that the split destinations receive light beams having light levels substantially equal to each other.
A bus system optical network system is disclosed in JP-A-1-222533. The network system includes two optical fiber cables for trunk transmission line, which are interconnected at one ends to each other, and an optical two-splitting coupler for connecting an output signal from a communication control unit and an input signal to the communication control device to the two optical fiber cables, respectively.
An optical splitter and an optical network are disclosed in JP-A-2001-166177. The optical splitter includes a light-transmission medium having first and second end faces, and a light diffusion portion provided at the first end face. The optical splitter changes a splitting ratio of an optical signal at an output portion in a manner that a light intensity distribution at an end face of a light emitting side of the light-transmission medium is changed by varying any of a distance between the first and second end faces, light incident position, and light diffusion angle.
Meanwhile, in a system for transmitting an optical signal to a plurality of devices connected to an optical fiber, it is sometimes desired to distribute optical signals of desired levels to the devices. An example of such is a case where video information is transmitted from a video output device to a receiver in a room at each floor in a building such as a hotel. In a possible way to distribute video information from the video output device to each receiver, an optical transmitting unit of the video output device is connected to optical receiving units of the receivers by optical fibers, respectively. In this case, installed optical fibers equal in number to the receivers are required, resulting in increase of wiring cost. A possible way to cope therewith is that an optical coupler are installed at each floor, a light signal from the optical transmitting unit is sent to the optical couplers, and in turn the couplers each splits the optical signal and sends the split ones to the receivers. However, when the optical couplers split the optical signal, a signal level of the optical signal drops depending on the splitting number. Accordingly, it is impossible to send the optical signal of sufficient level to the light receiving unit of each receiver. This problem would arise not only in the above case, but also in a system for transmitting an optical signal to another device by way of the optical fiber.